1. Field of the Invention
The invention is related to plasma devices or reactors that are used for cleaning, sterilization, surface activation, etching and thin-film deposition, and in particular to a low-temperature compatible, wide-pressure-range plasma flow device.
2. Description of the Related Art
Plasmas have found wide application in materials processing. For example, plasmas play a key role in the manufacture of integrated circuits and other semiconductor products. Plasmas that are used in materials processing are generally weakly ionized, meaning that less than 1% of the molecules in the gas are charged. In addition to the ions, these plasmas contain reactive species that can etch and deposit thin films at rates up to about a micron per minute. The temperature in these weakly ionized gases is usually below 200° C., so that thermally sensitive substrates are not damaged.
In some cases, the ions produced in the plasma can be accelerated towards a substrate to cause directional etching of sub-micron features into the material. In other cases, the plasma is designed so that most of the ions are kept away from the substrate leaving mainly neutral chemical species to contact it. Here, the goal is to isotropically etch the substrate, such as in the stripping of photoresist from silicon wafers. For a general description of weakly ionized plasmas, see Lieberman and Lichtenberg, “Principles of Plasma Discharges and Materials Processing”, (John Wiley & Sons, Inc., New York, 1994).
An important application of plasmas is the chemical vapor deposition (CVD) of thin films. The plasma enhances the CVD process by providing reactive species which attack the chemical precursors, causing them to decompose and deposit the material at a much lower temperature than is otherwise possible by thermal activation. See for example, Patrick, et al., “Plasma-Enhanced Chemical Vapor Deposition of Silicon Dioxide Films Using Tetraethoxysilane and Oxygen: Characterization and Properties of Films”, J. Electrochem Soc. 139, 2604-2613 (1992). In most applications, the ions are kept away from the chemical precursors as much as possible, because the ions may cause non-selective decomposition with the incorporation of unwanted impurities into the CVD film. In some applications, the ions are mixed with the precursors to provide a specialized process whereby the film is slowly etched at the same time it is deposited. This configuration can be useful for depositing material deep inside sub-micron trenches. However, in this case, ion-induced damage of the substrate may occur.
The literature teaches that weakly ionized plasmas are generated at low gas pressures, between about 0.001 to 1.0 Torr, by the application of radio-frequency (RF) power to a conducting electrode (see Lieberman and Lichtenberg (1994)). Sometimes microwave power is used instead of RF. The electrode may be designed to provide either capacitive or inductive coupling to strike and maintain the plasma. In the former case, two solid conducting electrodes are mounted inside a vacuum chamber, which is filled with the plasma. One of these electrodes is powered, or biased, by the RF generator, while the other one is grounded. In the latter case, the RF power is supplied through an antenna that is wrapped in a coil around the insulating walls of the vacuum chamber. The oscillating electric field from the coil penetrates into the gas inducing its ionization. U.S. Pat. No. 5,865,896 to Nowak, et al. Feb. 2, 1999) gives an example of such a design.
The substrate or work piece that is being treated by the plasma sits on a pedestal mounted inside the vacuum chamber. The pedestal may be grounded or at a floating potential, or may be separately biased from the RF powered electrode or antenna. The choice depends on the application (see Nowak et al. (1999)). There are also applications in which the electrodes are suspended away from the substrate or work piece so as to minimize contact with the ions. In these cases, the plasma is operated at pressures near 1.0 to 10.0 Torr, where the reactive neutral species exhibit much longer lifetimes in the plasma than the ions.
A disadvantage of plasmas operating at low pressures is that the concentration of reactive species can be too low to give the desired etching or deposition rate. For example, it has been shown by Kuo (“Reactive Ion Etching of Sputter Deposited Tantalum with CF4, CF3Cl and CHF3”, Jpn. J. Appl. Phys. 32, 179-185 (1993)) that sputter deposited tungsten films are etched at a maximum of 0.22 microns per minute, using 100 mTorr carbon tetrafluoride at 60° C. Rates at ten times higher than this are desirable for commercial manufacturing operations. Another disadvantage of low-pressure plasmas is that they are difficult to scale up to treat objects that are larger than about a square foot in area. The flux of ions and other reactive species to the substrate or work piece is a sensitive function of the density of charged particles in the plasma. The plasma density at any point within the vacuum chamber depends on the local electric field. This field is sensitive to the shape and composition of the vacuum chamber, the shape and composition of the work piece and the pedestal that holds it, the design of the electrode or antenna, and many other factors. Therefore, designing a plasma reactor requires many hours of engineering and experimentation, all of which greatly adds to the cost of the device.
A further disadvantage of low-pressure plasmas is that the reactive gas fills the entire volume inside the vacuum chamber. In these devices, it is impossible to completely separate the ions from the neutral reactive species. Ions always impinge on the substrate, and may cause damage, if, for example, it contains sensitive electronic devices, such as solid-state transistors. The ions and reactive gases may also damage the chamber and other system components, including the substrate holder, the gas injection rings, the electrodes, and any quartz dielectric parts. In plasma-enhanced chemical vapor deposition reactors, the films are deposited all over the inside of the chamber. These deposits alter the characteristics of the plasma as well as lead to particulate contamination problems. Consequently, plasma CVD reactors must be cleaned periodically to eliminate these residues. These deposits can be removed by introducing an etchant gas, such as NF3, into the chamber and striking a plasma. However, the residues are of different thickness and their rates of etching may not be uniform, making it difficult to satisfactorily clean all the surfaces. See Nowak et al. (1999). Ultimately, the CVD reactor must be taken out of service, cleaned by hand and the damaged parts replaced. These cleaning operations add to the cost of operating the plasma device and are a significant disadvantage.
Thus, there is a need for a plasma device that can provide higher fluxes of reactive species to increase etching and deposition rates, that is easily scaled up to treat large areas, that if needed, can eliminate the impingement of ions onto the substrate or work piece, and that confines the reactive gas flux primarily to the object being treated. The latter property would reduce the wear and tear on the device, and greatly reduce the need for reactor cleaning.
One way to increase the flux of reactive species in a plasma is to increase the total pressure. In this regard, several plasma devices have been developed for operation at atmospheric pressure. A discussion of these sources is given in Schutze et al., IEEE Transactions on Plasma Science, Vol. 26, No. 6, 1998, pp. 1685-1694, which is incorporated by reference herein. While these devices can provide high concentrations of reactants for etching and deposition, they have other disadvantages that make them unsuitable for many materials applications. The most common atmospheric-pressure plasma is the torch, or transferred arc, which is described by Fauchais and Vardelle, in their article: “Thermal Plasmas”, IEEE Transactions on Plasma Science, 25, 1258-1280 (1997). In these devices, the gas is completely ionized and forms an arc between the powered and grounded electrodes. The gas temperature inside the arc is more than ten thousand degrees Centigrade. This device may be used for processing materials at high temperatures, such as in metal welding, but is not useful for etching and depositing thin films as described in the preceding paragraphs.
To prevent arcing and lower the gas temperature in atmospheric-pressure plasmas, several schemes have been devised, such as the use of pointed electrodes in corona discharges and insulating inserts in dielectric barrier discharges. See Goldman and Sigmond, “Corona and Insulation,” IEEE Transactions on Electrical Insulation, EI-17, no. 2, 90-105 (1982) and Eliasson and Kogelschatz, “Nonequilibrium Volume Plasma Chemical Processing”, IEEE Transactions on Plasma Science, 19, 1063-1077, (1991). A drawback of these devices is that the plasmas are not uniform throughout the space between the electrodes. In addition, they do not produce the same reactive chemical species as are present in low-pressure plasmas of similar gas composition.
A cold plasma torch described by Koinuma et al. in their article: “Development and Application of a Microbeam Plasma Generator,” Appl. Phys. Lett., 60, 816-817 (1992). This device operates at atmospheric pressure, and can be used to etch or deposit thin films. In the cold plasma torch, a powered electrode, consisting of a metal needle 1 millimeter (mm) in thickness, is inserted into a grounded metal cylinder, and RF power is applied to strike and maintain the plasma. In addition, a quartz tube is placed between the cathode and anode, which makes this device resemble a dielectric barrier discharge. An atmospheric-pressure plasma jet is described by Jeong et al., “Etching Materials with an Atmospheric-Pressure Plasma Jet,” Plasma Sources Science Technol., 7,282-285 (1998), and by Babayan et al., “Deposition of Silicon Dioxide Films with an Atmospheric-Pressure Plasma Jet,” Plasma Sources Science Technol., 7, 286-288, (1998), as well as in U.S. Pat. No. 5,961,772 issued to Selwyn, all of which are incorporated by reference herein. The plasma jet consists of two concentric metal electrodes, the inner one biased with RF power and the outer one grounded. This device uses flowing helium and a special electrode design to prevent arcing. By adding small concentrations of other reactants to the helium, such as oxygen or carbon tetrafluoride, the plasma jet can etch and deposit materials at a low temperature, similar to that achieved in low-pressure capacitively and inductively coupled plasma discharges. The cold plasma torch and the plasma jet provide a beam of reactive gas that impinges on a spot on a substrate. These designs have a serious drawback in that they do not treat large areas uniformly. Scaling them up to cover larger areas, such as a square foot of material, is not straightforward and may not be possible. The operation of these plasma devices at pressures other than one atmosphere of pressure has not been described.
Thus, there is a need for a plasma device that operates at pressures ranging from 10.0 to 1000.0 Torr (1.0 Atmosphere=760 Torr), that can provide higher fluxes of reactive aspecies to increase etching and deposition rates, that is easily scaled up to treat large areas, that if needed, can eliminate the impingement of ions onto the substrate or work piece, and that confines the reactive gas flux primarily to the object being treated.